Strong intramolecular hydrogen bond in triformylmethane ab-initio, AIM and NBO study

The strong intramolecular hydrogen bond in triformylmethane was studied with ab-initio calculations. All possible conformations of the two tautomeric structures (keto and enol) of triformylmethane were fully optimized at HF, MP2 and B3LYP levels with 6-31G** basis set in order to determine the conformational equilibrium. These calculations were compared with the result of HF, MP2 and B3LYP methods with most extended (6-311++G**) basis set. In general the chelated enol structures, T1 and C1, are more stable than the other conformers and all of these levels predict that the T1 structure is a global minimum. This result is in contrast with the conclusion from the microwave study, which supports the C1 conformer as the most stable form. Furthermore, to have more reliable energies in the case of the more stable conformers, their total energies were recomputed at the G2(MP2) level. Although, the electron correlation effect has an important effect on the stability order of conformers but ZPVE is not important. Our theoretical calculations show that the hydrogen bond strength increases on going from MA to C1 and T1 (MA